The present invention broadly relates to the papermaking art, and, in particular, concerns a new and improved construction of a headbox for a papermaking machine.
Generally speaking, the headbox of the present development is of the type comprising a distributor device rheologically connected with an infeed line or conduit for a fiber stock suspension and a nozzle channel merging with and rheologically connected to such distributor device in the flow direction of the fiber stock suspension. The nozzle channel is formed between two lip members extending over a predetermined working or operating width of the headbox. These lip members bound a slice opening or outlet gap for forming a substantially flat or areal fiber stock jet. At least one of both lip members is movably connected with the distributor device and is operatively coupled with an adjustment device, by means of which there can be adjusted the flow downstream located end section of this lip member in the sense of an adjustment of its spacing from the oppositely located lip member, and such positionally adjusted lip member can then be fixed in its adjusted position.
With a known headbox of the aforementioned type, for instance as disclosed in the German Pat. No. 2,736,644, the nozzle channel is bounded by a lower lip member which is fixedly connected with the distributor device and an upper lip member which is adjustable relative to the lower lip member and can be fixed in its adjusted position. This upper lip member is constructed as a one-piece member essentially over the entire length of the nozzle channel measured in the flow direction of the stock suspension and is hingedly mounted at the region of the distributor device. At the flow downstream end of the upper lip member there is hingedly connected a pre-lip which covers the web forming zone of the papermaking machine located externally of the nozzle channel.
With this known apparatus the movable upper lip member is pivotable as a unit about its flow upstream located pivot axis by means of its adjustment mechanism. The momentary position of the flow downstream located end of this upper lip member with respect to the lower lip member governs the height of the outlet gap or slice opening. The pattern of the cross-sectional area of the nozzle channel considered in the flow direction of the stock suspension, for a given height of the slice opening, is essentially unalterable by virtue of the once selected installed position of both lip members. This can lead to difficulties in papermaking machines which should be operated, for instance, with various types of stock suspensions at different times. This is so because the nozzle channel which has been optimized for the processing of a certain quality of the fiber stock, for instance a suspension of short fiber material, generally is unsuited or only insufficiently suitable for the processing of a different quality of the fiber stock, for instance a suspension of long fiber material.
To achieve optimum operating conditions at the papermaking machine it is therefore necessary to accommodate the dimensional shape and size of the nozzle channel to the different flow velocities, the acceleration conditions with respect to the fiber orientation and/or different residence times of the stock suspensions in the nozzle channel, which must be prescribed as a function of the type of fiber stock which is momentarily being processed. With heretofore known constructions of headboxes having essentially unalterable geometry of the nozzle channel such type of accommodation of the headbox to the aforementioned parameters required relatively complicated conversion work at the headbox, resulting in correspondingly long downtimes of the papermaking machine.